I have no problem in my internet connection. But getting every time this screen when I try to update netbeans plugins or IDE itself.
Check your Netbeans log - messages.log file - for details of the error!
Example locations of the file:
Linux: ~/.netbeans/7.3/var/log/messages.log
Windows: c:\Users\USERNAME\AppData\Roaming\NetBeans\7.3\var\log\